Devices and methods for non-invasive implant length sensing

ABSTRACT

A device for the non-invasive sensing of the length of an implantable medical device includes an implantable medical device having first and second portions moveable relative to one another and a layer of resistive material disposed on one of the first and second portions. A contact is disposed on the other of the first and second portions, the contact being in sliding contact with the layer of resistive material upon relative movement between the first and second portions. A circuit is configured to measure the electrical resistance along a path including a variable length region of the layer of resistive material and the contact. The electrical resistance can then be converted into a length.

FIELD OF THE INVENTION

The field of the invention generally relates to implantable medicaldevices and more particularly, implantable medical devices that undergochanges in length.

BACKGROUND

A variety of medical devices exist that are implanted inside the bodyand undergo a dimensional change. For example, a bone lengthening deviceis one type of implantable device that is typically inserted into firstand second portions of a severed or broken bone. The device is thenperiodically lengthened to distract or grow the bone over a period oftime. Such adjustments made to the bone lengthening device may beinvasive or even non-invasive. As another example, growing rods ordistraction devices may be secured to a subject's spine. These devicesmay be used to correct a medical condition such as scoliosis. In stillother applications, these devices may be used to increase the distancebetween adjacent vertebrae to reduce symptoms associated with lumbarspinal stenosis or pinched nerves. Other bones such as the jaw bone mayinclude an implantable medical device that is configured to elongateover time.

Regardless of the nature in which the implanted medical device is used,there often is a need to determine the absolute length of the implant asit exists inside the patient at any given moment. As an example, afterthe implanted medical device has undergone a length adjustment there isa need to determine whether or not the desired quantity of lengtheningwas indeed achieved. In addition, devices may change dimensions afterthe adjustment has been made (whether manual or non-invasive). Forexample, normal physiological movement of the subject may causeadditional shortening or lengthening of the device after adjustment ofthe device. In these instances, it would be beneficial to know theactual length of the device in between adjustment procedures. Forexample, a physician might want to know if the device has strayed toofar in either direction to warrant an additional adjustment.

U.S. Patent Application Publication No. 2010/0094302 discloses anon-invasive medical implant device that uses microphone sensor on anexternal adjustment device to sense when an internally-located magnet isundergoing rotation. Specifically, the microphone sensor picks up anacoustic signal (e.g., click) that is periodically generated by rotationof an internal magnet that is part of the implantable medical device. Bycounting the number of clicks, the external adjustment device can thentranslate this into an estimated length of the device. While such amethod does provide a means to determine the length of the implantedmedical device there is the possibility that one or more of the clicksignals may not be detected by the external adjustment device. In thisinstance, the actual length of the implanted medical device may then bedifferent from the length that is calculated or otherwise determined bythe external adjustment device. Further, while the external adjustmentdevice may store the most current length of the device as determined bythe sensed signals, it is possible that the subject may return to adifferent physician for his or her next adjustment procedure. Unless thesize of the implanted medical device is stored locally on or with thepatient (e.g., RFID or a card carried by the patient), the nextphysician will not know the most recent sizing of the device. Moreover,as stated above, there is the possibility that the implanted device maychange lengths in between adjustment procedures. There thus is a needfor methods and devices that will determine the absolute length of animplantable medical device at any given movement.

SUMMARY

In one embodiment of the invention, a device includes an implantablemedical device having first and second portions moveable relative to oneanother and a layer of resistive material disposed on one of the firstand second portions. The device includes a contact disposed on the otherof the first and second portions, the contact being in sliding contactwith the layer of resistive material upon relative movement between thefirst and second portions and a circuit configured to measure theelectrical resistance along a path including a variable length region ofthe layer of resistive material and the contact.

In another embodiment of the invention, a method of sensing the lengthof an implantable medical device having first and second portionsmoveable relative to one another is disclosed. The implantable medicaldevice includes a resistive pathway on one of the first and secondportions of the implantable medical device along with a contact disposedon the other of the first and second portions, the contact being insliding contact with the resistive pathway upon relative movementbetween the first and second portion. The electrical resistance along apath including a variable length region of the resistive pathway and thecontact is measured. The measured electrical resistance is thenconverted to a length.

In another embodiment, a device includes an implantable medical devicehaving first and second portions moveable relative to one another and aprimary drive coil surrounding a segment of the implantable medicaldevice containing both the first and second portions. At least onesecondary coil surrounds a segment of the implantable medical devicecontaining both the first and second portions. The device includes afirst circuit configured to apply a drive voltage to the primary drivecoil and measure signals in the at least one secondary coil and output asignal indicative to the length of the implantable medical device.

In another embodiment, a method of sensing the length of an implantablemedical device having first and second portions moveable relative to oneanother includes applying a driving voltage to a primary drive coilsurrounding a segment of the implantable medical device containing boththe first and second portions. Signals in at least one secondary coilsurrounding a segment of the implantable medical device containing boththe first and second portions are measured and converted to a length.

In another embodiment, a device includes an implantable medical devicehaving first and second portions moveable relative to one another,wherein the first and second portions are separated from one another bya dielectric material. The device includes an implantable resonant coilcoupled to the first and second portions and an externally located drivecoil operatively coupled to a signal generator. A frequency analyzer isoperatively coupled to the drive coil configured to detect the resonantfrequency of the implantable medical device, wherein said resonantfrequency varies depending on the degree of relative orientation betweenthe first and second portions.

In another embodiment a method of sensing the length of an implantablemedical device having first and second portions moveable relative to oneanother includes providing an implantable medical device wherein thefirst and second portions are coupled to an implanted resonant coil. Anexternal drive coil is driven adjacent to the implanted resonant coilwith a signal generator at different frequencies. A resonant frequencyof the implantable medical device is detected and the resonant frequencyof the implantable medical device is converted to a length.

In another embodiment, a device includes an implantable medical devicehaving first and second portions moveable relative to one another and anelongate member having first and second ends, the first end beingsecured to the first portion of the implantable medical device, thesecond end having secured thereto a magnet. The device includes afulcrum on the second portion of the implantable medical device andcontacts various points along the elongate member in response torelative movement of the second member relative to the first member. Anexternally located magnetic field source is configured to apply anoscillating magnetic field in proximity to the magnet secured to theelongate member.

In another embodiment, a method of sensing the length of an implantablemedical device includes providing an implantable medical device havingfirst and second portions moveable relative to one another, an elongatemember having first and second ends, the first end being secured to thefirst portion of the implantable medical device, the second end havingsecured thereto a magnet, and a fulcrum on the second portion of theimplantable medical device and in contact with various points along theelongate member in response to relative movement of the second memberrelative to the first member. An oscillating magnetic field is appliedat different frequencies in proximity to the magnet with an externallylocated magnetic field source driven by a power source. The power sourcefor the externally located magnetic field source is monitored whereinthe resonant frequency of the elongate member is determined based atleast in part on the current draw of the power source. The resonantfrequency of the elongate member is then converted to a length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a device that non-invasively measures the length ofan implantable medical device according to one embodiment.

FIG. 1B illustrates a full sectional view of the device of FIG. 1A takenalong the line A-A.

FIG. 1C illustrates view B of FIG. 1B.

FIG. 1D illustrates a calibration curve for the device of FIGS. 1A-1C.

FIG. 2A illustrates a perspective view of an external adjustment device.

FIG. 2B illustrates a perspective view of an external adjustment deviceaccording to another embodiment.

FIG. 3A illustrates a device that non-invasively measures the length ofan implantable medical device according to another embodiment.

FIG. 3B illustrates a calibration curve for the device of FIG. 3A.

FIG. 4A illustrates a device that non-invasively measures the length ofan implantable medical device according to another embodiment.

FIG. 4B illustrates a calibration curve for the device of FIG. 4A.

FIG. 4C illustrates another embodiment of the device of FIG. 4A.

FIG. 5A illustrates a device that non-invasively measures the length ofan implantable medical device according to another embodiment.

FIG. 5B illustrates an enlarged view showing the rod and fulcrum thatinterfaces with the elongate member having a magnet disposed on an endthereof.

FIG. 5C illustrates a cross-sectional view taken along the line B-B ofFIG. 5B.

FIG. 5D illustrates a motor driven external magnet for use with thedevice of FIG. 5A.

FIG. 5E illustrates an external electromagnet for use with the device ofFIG. 5A.

FIG. 5F illustrates a calibration curve for the device of FIG. 5A.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1A illustrates a device 10 that non-invasively measures the lengthof an implantable medical device 12. The implantable medical device 12may include any number of implantable medical devices 12 including thoseused to lengthen or distract bone or other tissue. These include, by wayof example, distraction devices for use in bone lengthening applications(e.g., limb lengthening), spinal distraction devices for the treatmentof scoliosis, spinal devices for the treatment of lumbar spinalstenosis, dental devices, and the like. The implantable medical device12 may be one that is adjusted manually or more preferably,non-invasively. Still referring to FIG. 1A, the implantable medicaldevice includes a first portion 14 and a second portion 16 that aremoveable relative to one another. During use, the first portion 14 andsecond portion 16 move apart from one another for distraction orlengthening. Conversely, the first portion 14 and the second portion 16may move toward one another for adjustment purposes or even to reducelengthening or distraction forces. As seen in FIGS. 1A-1C, the first andsecond portions 14, 16 move in a telescopic fashion. In otheralternative embodiments, however, the first and second portions 14, 16do not need to be fashioned to move relative to one another in atelescopic manner. During implantation, the first portion 14 istypically secured to a first location (e.g., bone) while the secondportion 16 is secured to a second, different location (e.g., differentbone). Various fixation devices such as screws, pins, hooks,articulating joints, and the like may be used to secure one or both endsof the first and second portions 14, 16. In other embodiments, such aslimb lengthening applications, a cavity, which may be natural or formedinside the bone, is used to receive one or both of the first and secondportions 14, 16. The first and second portions 14, 16 may be securedusing a mechanical fastener, cement, or method commonly known to thoseskilled in the art.

In the embodiment illustrated in FIGS. 1A-1C, the first portion 14 is ahousing 18 in the shape of a tube or the like and is made from abiocompatible metal such as titanium. The housing 18 includes therein apermanent magnet 20 that is configured for rotational movement relativeto the housing 18. The permanent magnet 20 may be formed from a rareearth magnet such as Neodymium-Iron-Boron. The permanent magnet may bemade from a grade of N35 or higher, for example a grade of N50. One ormore bearings 22 may be used to retain the permanent magnet 20. A leadscrew 24 is connected either directly or indirectly via gears or thelike (not shown) to the permanent magnet 20. Thus, rotational movementof the permanent magnet 20 causes rotational movement of the lead screw24.

The second portion 16 of the implantable medical device 12 is in theform of a rod 26 that includes a hollow segment 28 that is dimensionedto receive the lead screw 24. A nut 30 is located within the hollowsegment 28 and has threading that interfaces with threads located on thelead screw 24. Rotation of the lead screw 24 in a first direction thuscauses the rod 26 to telescope into the housing 18 thus shortening theoverall length of the implantable medical device 12. Conversely,rotation of the lead screw 24 in a second, opposite direction causes therod 26 to telescope out of the housing 18 thus lengthening the overalllength of the implantable medical device 12. A seal 32 is providedbetween the rod 26 and the housing 18 such that fluids and materialremain external to the rod 26 and housing 18 during movement.

Rotation of the permanent magnet 20 is accomplished by application of amoving magnetic field from a location external to the subject having theimplantable medical device. The moving magnetic field is typicallyapplied using an external adjustment device that has one or morerotating magnets that generate the driving magnetic field. Particulardetails on the nature of the external adjustment devices that can beused in connection with the distraction devices described herein aredisclosed, for example, in U.S. Patent Application Publication Nos.2009/0112207, 2010/0094302, 2010/0121323, and U.S. patent applicationSer. No. 13/172,598, all of which are incorporated by reference herein.FIG. 2A illustrates an external adjustment device 400 according to oneembodiment that includes two permanent magnets 402, 404 contained withinrespective covers 406. Each permanent magnet 402, 404 is rotatablewithin its respective cover 406 and provides a moving magnetic field. Amotor 408 is mechanically engaged to the permanent magnets 402, 404 viaa transmission (not shown) contained within a housing 410 of theexternal adjustment device 400. FIG. 2B illustrates another embodimentof an external adjustment device 700. The external adjustment deviceincludes a first handle 702 and a second handle 704. Like the priorembodiment, there are two permanent magnets 706, 708 that are rotatableand provide a moving magnetic field. A power cord 711 supplies power toa motor 705 that drives the permanent magnets 706, 708 via a gear box(not shown). Additional details regarding external adjustment device 700may be found in U.S. application Ser. No. 13/172,598 which isincorporated by reference herein.

Referring back to FIGS. 1A-1C, the housing 18 includes a layer ofresistive material 36 on an inner surface thereof. As best seen in FIGS.1A and 1B, the housing 18 includes two separate layers of the resistivematerial 36 on generally opposing interior surfaces of the housing 18.It should be understood, however, that a single layer of resistivematerial 36 may suffice. Similarly, there may be more than two layers ofresistive material 36. The layer of resistive material 36 may includeany number of materials known to have a degree of electrical resistance.Exemplary materials include, for example, carbon although other knownelectrically resistive materials will work. Generally, in order toincrease the resolution of the device 10, the resistive material 36should have a relatively high degree of electrical resistance so thatsmall changes in length will correspond to larger resistancedifferentials. An electrically conductive contact 38 is secured to theend of the rod 26. The contact 38 thus moves in conjunction with the rod26 upon actuation of the permanent magnet 20. The contact 38 includescontact surfaces 40 that physically touch the resistive material 36. Thecontact surfaces 40 may include an edge, brushes, or rollers thatcontact the layer of resistive material 36. In this regard, the contact38 forms part of the electrical circuit for measuring the resistance ofthe implantable medical device 12. Depending on the location of the rod26, the actual length of the resistive material 36 within the electricalcircuit varies. Thus, when the rod 26 is extended outward with respectto the housing 18 a larger portion of the resistive material 36 is partof the resistance circuit and resistance measurements via the circuit(discussed below in more detail) are higher. Conversely, when the rod 26is collapsed inside the housing 18 a smaller portion of the resistivematerial 36 is part of the resistance circuit and resistancemeasurements via the circuit are lower. The contact 38 includes anaperture 42 therein for passage of the lead screw 24.

As seen in FIG. 1A, electrical conductors 44, 46 electrically connect tothe two respective layers of resistive material 36. These electricalconductors 44, 46 connect to a circuit 48 which may be implemented in amicroprocessor or the like. For example, the circuit 48 may include oneor more processors configured to read and process measured resistancevalues at points a and b of the circuit in FIG. 1A. The circuit 48 mayalso optionally include memory for storing data therein. The memory maycomprise an RFID chip. The data may include, for example, resistancevalues or corresponding data as well as calibration data unique to theparticular implantable medical device 12. The circuit 48 is coupled toan antenna 50 which enables the circuit 48 to transmit data and otherinformation to an externally located controller 52. The externallylocated controller 52 includes an antenna 54 and acts as a receiver toreceive data communicated from the circuit 48. Thus, the circuit 48 actsas a transmitter while the controller 52 acts as a receiver. In oneaspect of the invention, the controller 52 both powers and communicatesdigitally with the circuit 48. In this regard the circuit 48 does notneed its own power source as the circuit 48 is powered inductively viathe external controller 52. As seen in FIG. 1A, the communication occursacross the skin 56 of the subject.

The controller 52 includes a display 58 that is used to display one ormore parameters indicative of the length of the implantable medicaldevice 12. This can include a numerical value corresponding to theabsolute length of the implantable medical device 12. The display 58 mayalso include a graphical representation of the device length (e.g., bargraph or the like) or other indicia of length. The controller 52 mayalso display the resistance value measured by the circuit 48.

FIG. 1B illustrates an optional feature wherein a plurality oflongitudinal grooves 60 are disposed along an inner surface of thehousing 18. The rod 26 includes corresponding protrusions (not shown)that form a splined tip that interfaces with the corresponding grooves60. The tight tolerance of the splined tip with the grooves 60 keeps therod 26 centered within the housing 18. Further, the combination of thesplined tip and the grooves 60 act as an anti-rotation feature thatprevents the rod 26 from rotating relative to the housing 18. Additionaldetails regarding this optional aspect may be found in U.S. PatentApplication Publication No. 2010/0217271 which is incorporated herein byreference.

FIG. 1C illustrates an enlarged view of detail B of FIG. 1B. As seen inFIG. 1C, the layer of resistive material 36 may be disposed on aninsulation layer 62. The insulation layer 62 electrically insulates thelayer of resistive material 36 from the underlying metallic housing 18.The insulation layer 62 may include a thin oxide layer that is formed byanodization.

To use the device 10, the external adjustment device 400 is placed nearor adjacent to the patient's skin 56 near the location of implantablemedical device 12. Activation of the external adjustment device 400causes the two permanent magnets 402, 404 to rotate thereby causing thepermanent magnet 20 to rotate along with the lead screw 24. Depending onthe direction of rotation, the rod 26 either extends from or retractsinto the housing 18. The actual length of the implantable medical device12 is determined by measuring the resistance at points a and b using thecircuit 48. The resistance measured at points a and b varies dependingon the position of the contact 38 on the layer of resistive material 36.As the contact 38 moves away from the permanent magnet 20 additionalresistance is added to the electrical pathway because of the additionalresistive material 36 that is present in the electrical circuit. Thecircuit 48 measures this resistance value wherein it can be stored inthe circuit 48 for later transmission or, alternatively, the resistancevalue may be directly transmitted to the controller 52. For example, inone aspect of the invention, when the circuit 48 is powered inductivelyvia the external controller 52, one or more resistance measurements aremade by the circuit 48 and this information is then transmittedwirelessly to the controller 52. The controller 52 can then take thisdata (e.g., resistance data) and convert the same to a length. Forexample, the controller 52 may include calibration data (calibrationcurve, look-up table, or the like) that is used to translate aresistance value into a length. The length can then be displayed on thedisplay 58. The calibration data may be communicated from the circuit 48or it may have already been pre-loaded into the controller 52. Inaddition, it is possible that the circuit 48 itself translates theresistance reading into a length and data corresponding to a length istransmitted to the controller 52.

FIG. 1D illustrates an illustrative calibration curve for the device ofFIGS. 1A-1C. As seen in FIG. 1D, an increased resistance corresponds toan increased length of the implantable medical device 12. Thecalibration curve may be used for a number of different implantablemedical devices 12 or it may be tailored to a single implantable medicaldevice 12. In the example of FIG. 1D, a reading of 146 Ω corresponds toa length of 25.0 mm. Once this conversion has been made the length of25.0 mm can then be displayed on the display 58 of the controller 52.

FIG. 3A illustrates an alternative embodiment of a device 70 thatnon-invasively measures the length of an implantable medical device 72.The implantable medical device 72 includes a first portion 14 and asecond portion 16 that are moveable relative to one another. Thoseaspects of this embodiment of the implantable medical device 72 that arecommon with the embodiment of FIGS. 1A-1C are labeled with the sameelement numbers and will not be described again. In this embodiment, themagnetic coupling between coils is used to measure the length of theimplantable medical device 72. In a similar manner to a linear variabledifferential transformer (LVDT) a primary coil 74 circumscribes thehousing 18 and is coupled to a drive circuit 76 that deliversalternating current through the primary coil 74. A secondary coil 78 ormultiple secondary coils connected in reverse series also surround thehousing 18 and are coupled to a sensing circuit 80 which may be the sameas or different from the drive circuit 76. The output signal from thesecondary coil(s) 78, which is typically a voltage, is generallyproportional to the distance moved by the rod 26 within the housing 18.The location of the primary coil 74 and the secondary coil 78 is suchthat the coils 74, 78 generally circumscribe the region of overlapbetween the rod 26 and the housing 18. By measuring the output signalwith the secondary coil(s) 78, this value can then be translated into alength of the implantable medical device 72.

The sensed or decoded signal received from the secondary coil(s) 78 isthen passed to a telemetry circuit 82. The telemetry circuit 82wirelessly transmits data through the skin 56 via an antenna 84 to anexternal controller 86. The external controller 86 is includes anantenna 88 and acts as a receiver to receive data communicated from thetelemetry circuit 82. Thus, the telemetry circuit 82 acts as atransmitter while the controller 86 acts as a receiver. In one aspect ofthe invention, the controller 86 both powers and communicates digitallywith the telemetry circuit 82. The controller 86 may also power thesensing circuit 80. In this regard the circuits 80, 82 do not need theirown power source as the circuits 80, 82 are powered inductively via theexternal controller 86. As seen in FIG. 3A, the communication occursacross the skin 56 of the subject.

The controller 86, like the controller 52 of the earlier mentionedembodiment, has a display 90 that is used to display length informationto a user. In one aspect of the invention, the controller 86 convertsthe data transmitted by the telemetry circuit 82, which may be voltagedata, into length data. The controller 86 may do this by usingcalibration data that relates the degree of magnetic coupling (e.g.,voltage output) to a length. FIG. 3B illustrates an example of acalibration that relates output (voltage) as a function of length. Thiscalibration data may be stored in the controller 86 or transmitted fromthe telemetry circuit 82. Alternatively, the conversion from magneticcoupling to length can be done by the sensing circuit 80. Regardless ofwhere the conversion is made, once a length is determined, the value canbe presented to the user on the display 90.

In this embodiment, the rod 26 acts as a core that affects the degree ofmagnetic coupling between the primary coil 74 and the secondary coil 78.The rod 26 is preferably made from a material with a relatively highdegree of magnetic permeability. This may include metals or alloys ofmetals. The material should also be biocompatible. Titanium and titaniumalloys have excellent biocompatibility and marginal magneticpermeability. To improve on the efficiency of the resulting LVDT, a rodmaterial with a higher magnetic permeability, such as stainless steel orother iron containing materials may be used. If a titanium or titaniumalloy rod is desirable, an additional core component with highermagnetic permeability may be attached to the rod. For example a tube maybe secured over the outer diameter or within an inner diameter of therod 26. Alternatively, the nut 30 may be made from a material withhigher magnetic permeability than titanium, and thus improve the effectof the core. Generally, the higher degree of magnetic permeabilityshould translate into a device 70 with more sensitivity because anincremental movement of the rod 26 relative to the housing 18 willresults in a larger change in magnetic coupling. That is to say, in thecontext of the calibration curve of FIG. 3B, for a given change inlength, a higher degree of magnetic permeability would translate into alarger change in voltage output.

FIG. 4A illustrates another alternative embodiment of a device 100 thatnon-invasively measures the length of an implantable medical device 102.The implantable medical device 102 includes a first portion 14 and asecond portion 16 that are moveable relative to one another. Thoseaspects of this embodiment of the implantable medical device 102 thatare common with the embodiment of FIGS. 1A-1C are labeled with the sameelement numbers and will not be described again. In this device 100, aresonant coil 104 is electrically coupled to the housing 18 and the rod26 via electrical conductors 106, 108. The electrical conductors 106,108 may include wires, electrical traces in a printed circuit board, orthe like. The rod 26 and the housing 18 are electrically isolated fromone another. Electrical isolation may be accomplished by preventing thephysical touching of the two components. In addition, electricalisolation may be achieved by coating one or both of the rod 26 andhousing 18 with a dielectric material. This will isolate the rod 26 fromthe housing 18 and create a variable capacitor. A dielectric layer 27between the rod 26 and housing 18 forms a capacitor that varies as therod 26 moves in and out of the housing 18. The movement of the rod 26changes the surface area of overlap between the rod and the housing thuschanging the capacitance as a function of implant length. When the rodis fully retracted, the capacitance is maximized. When the rod is fullyextended, the capacitance is minimized. The dielectric layer 27 mayconsist of a thin Kapton® (polyimide) tube bonded to the inner diameterof the housing 18, although other dielectric materials may be used(e.g., a gas such as air). Thus, the dielectric layer 27 may be appliedas a layer or coating on either the rod 26 or the housing 18 (or both).Alternatively, the dielectric layer 27 may be a gap between the rod 26and the housing 18 that is filled with air. The resonant coil 104connected to the capacitor acts as an inductor, creating a resonant LCcircuit. The device 100 further includes a drive coil 110 that iscoupled to a signal generator 112. The drive coil 110 and signalgenerator 112 are located external to the subject with the implantablemedical device 102 as illustrated in FIG. 4A. The signal generator 112generates an alternating current (AC) signal that delivered to the drivecoil 110. The drive coil 110 is located near or adjacent to the skin 56of the subject at a location that is near or adjacent to the resonantcoil 104. The resonant coil 104 may be located at a location remote fromimplantable medical device 102 (e.g., near the surface of the skin 56).The signal generator 112 preferably has the ability to deliver ACsignals to the drive coil 110 at a variety of frequencies via input 114.The device 100 further includes a frequency analyzer 116 which may takethe form of an oscilloscope or the like. The frequency analyzer 116 isused to determine when the resonant frequency of the implantable medicaldevice 102 has been reached. In particular, the frequency of the appliedAC signal is adjusted via the signal generator 112 until the frequencyanalyzer 116 detects that the resonant frequency has been reached. Thefrequency range sweep by the signal generator 112 may be automated oreven manual. Detection is made when the amplitude of the signal detectedby the frequency analyzer 116 drops or dips significantly. This may bedetected automatically by the frequency analyzer 116. The user will thusknow the resonant frequency of the implantable medical device 102 whichcan then be converted to a length. The particular resonant frequency ofthe implantable medical device 102 varies as a function of the length. Acalibration curve that includes the resonant frequency of theimplantable medical device 102 as a function of length can then be useddetermine the absolute length of the implantable medical device 102. Inthis regard, once the user knows the resonant frequency, thecorresponding length value can be determined using the calibrationcurve. This can be done manually or it could be automated using aprocessor or computer that translates the measured resonant frequencyinto a length value. The frequency analyzer 116 may be separate from thesignal generator 112 as seen in FIG. 4A, however, in other embodimentsthese two components may be integrated into a single external device asseen in FIG. 4C.

Table 1 listed below illustrates data obtained generating a calibrationcurve of an implantable medical device 102. Data was obtained by varyingthe length of the implantable medical device 102 in ¼ inch incrementsand adjusting the frequency of the signal generator 112 until theresonant frequency was observed with the frequency analyzer 116. In thetested device, 0.0025 inch KAPTON® polyimide tape was wrapped around therod 26 to isolate it from the housing 18 and act as a dielectric. Asnoted above, the resonant frequency was determined based on a dip of theamplitude of the signal measured by the frequency analyzer 116.

TABLE 1 Distraction Resonant (inches) Frequency (kHz) 0 104.016 .25110.083 .50 116.780 .75 123.887 1.00 133.136 1.25 141.557 1.50 153.6351.75 168.469 2.00 190.366

FIG. 4B illustrates the data of Table 1 plotted as a calibration curveshowing distraction length as function of resonant frequency. In thisembodiment, once the resonant frequency is determined, one can thenreadily determine the distraction length based on the calibration curve.The calibration curve may be unique to the implantable medical device102 and may be provided with the same or the calibration curve can bedeveloped by the physician. Again, this method enables the physician orother skilled person to determine the absolute length of the implantablemedical device 102 by non-invasive interrogation.

FIGS. 5A-5E illustrate another alternative embodiment of a device 110that non-invasively measures the length of an implantable medical device112. The implantable medical device 112 includes a first portion 14 anda second portion 16 that are moveable relative to one another. Thoseaspects of this embodiment of the implantable medical device 102 thatare common with the embodiment of FIGS. 1A-1C are labeled with the sameelement numbers and will not be described again. As best seen in FIGS.5A and 5B, an elongate member 114 is disposed inside the housing 18 andis affixed to the housing 18 at one end thereof. On the opposing end ofthe elongate member 114 is located a permanent magnet 116 (best seen inFIG. 5B). The elongate member 114 passes through a fulcrum 118 that isfixedly attached to the rod 26. As best seen in FIGS. 5B and 5C, thefulcrum 118 may include a projection 120 extending from the rod 26 thatincludes an aperture 122 dimensioned to permit the fulcrum 118 to slidealong the length of the elongate member 114 as the rod 26 is movedrelative to the housing 18. In this regard, the fulcrum 118 adjusts thelength of the elongate member 114 between the magnet 116 and the fulcrum118. By changing this length, the natural vibrational frequency of theelongate member 114 changes.

As part of the device 110, an externally located magnetic field source124 is provided that applies an oscillating magnetic field to theimplantable medical device 112. The oscillating magnetic field passesthrough the skin 56 of the subject and interacts with the magnet 116disposed on the end of the elongate member 114. During use, thefrequency of the oscillating magnetic field is adjusted (e.g., step wiseadjustment) until the resonant frequency of the elongate member 114 isreached. At the resonant frequency, the magnet 116 and attached elongatemember 114 vibrate back-and-forth as illustrated by arrow A of FIG. 5B.This vibration can be detected by the power that is supplied to magneticfield source 124. In one aspect, as seen in FIG. 5D, the magnetic fieldsource 124 is a permanent magnet 125 that is rotated by a motor 126 thatis powered by a driving circuit 128. The driving circuit 128 is capableof adjusting the rotational frequency of the motor 126 and thus themagnet 125. The driving circuit 128 is also able to monitor the currentthat drives the motor 126 using current sense circuitry. The resonantfrequency is detected when a current spike is observed in the drivingcircuit 128. The frequency of the oscillating magnetic field can then beconverted to a device length by using a calibration curve that relateslength of the device to the rotational frequency of the oscillatingmagnetic field. FIG. 5F illustrates an exemplary calibration curve ofthe resonant frequency as a function of length for the device 110. Thelength of the device 110 may be determined by examining where theresonant frequency intersects with the calibration curve. As with allthe calibration curves described herein, this may be done as a look-uptable, function, or other method commonly known to those skilled in theart. As an alternative to the use of the motor 126, FIG. 5E illustratesan alternative embodiment that uses an electromagnet 130 that is drivenby a driving circuit 128. Driving current can be measured by currentsense circuitry in the driving circuit 128.

While embodiments have been shown and described, various modificationsmay be made without departing from the scope of the inventive conceptsdisclosed herein. For example, while the devices described in detailherein are driven non-invasively, the methods and devices are alsoapplicable to implantable medical device that are adjusted manually.Similarly, while embodiments described in detail herein utilize a magnetcoupled to a lead screw to adjust the length of an implantable medicaldevice other drive devices may fall within the scope of the invention.Moreover, only a portion of the implantable medical device may changelength or shape and be measured in a non-invasive manner. Theinvention(s), therefore, should not be limited, except to the followingclaims, and their equivalents.

What is claimed is:
 1. A device comprising: an implantable medical device having a first portion and a second portion that are telescopically moveable relative to one another, wherein the first portion includes a housing having a first closed end configured to be secured to a first portion of a bone and a second open end opposite the first closed end, a first permanent magnet within the housing, and a lead screw within the housing that extends along a central axis of the housing and is rotatably secured to the first permanent magnet, wherein the first permanent magnet is configured to receive an oscillating magnetic field from an external source to rotate the lead screw non-invasively, wherein the second portion includes a rod having a first closed end configured to be secured to a second portion of the bone, a hollow segment dimensioned to receive the lead screw, and a second open end opposite to the first closed end of the rod having a threaded nut configured to engage the lead screw within the hollow segment, wherein the first portion is configured to telescopically move relative to the second portion by rotating the lead screw; an elongate member disposed within the housing of the first portion of the implantable medical device along a longitudinal axis offset from the central axis of the housing, the elongate member including a first end secured to the housing adjacent to the first permanent magnet and a second end secured to a second permanent magnet opposite the first end; and a fulcrum coupled to the rod of the second portion of the implantable medical device, the fulcrum including a projection that extends radially from the rod, and an aperture in the projection that is dimensioned to permit the fulcrum to slide along a length of the elongate member extending therethrough in response to relative telescopic movement of the second portion relative to the first portion.
 2. The device of claim 1, further comprising: an externally located magnetic field source, the externally located magnetic field source configured to apply an oscillating magnetic field in proximity to the second permanent magnet secured to the elongate member.
 3. The device of claim 2, wherein the externally located magnetic field source comprises an electromagnet.
 4. The device of claim 3, further comprising current sense circuitry operatively coupled to the electromagnet.
 5. The device of claim 2, wherein the externally located magnetic field source comprises a rotatable permanent magnet operatively connected to a motor.
 6. The device of claim 5, further comprising current sense circuitry operatively coupled to the motor. 